The present invention relates generally to an optical information recording and reproducing memory system which records, reproduces or erases information data by means of the irradiation of a light beam onto a data storing medium.
More specifically, the present invention relates to the memory system wherein the light beam is focus controlled or tracking-controlled relative to the appropriate position of the data storing medium.
It is well known to effect information storing, reproducing or erasing onto an optical data storing medium such as a metal film, thermoplastics or magneto-optical magnetic film by means of an optical information recording and reproducing memory system.
In the above memory system, information data is optically or magneto-optically stored on or erased from the optical data storing medium by the formation of pits or by the phase transmuting of the film.
Therefore, when the memory system is operated in the data writing or erasing mode, a light source e.g. laser diode emits a high powered light beam so that the pit formation or phase transmuting can be performed on the data storing medium.
On the other hand, when the memory system is operated in the data reproducing mode (read-out mode), the light source emits a weakened light beam so that any destruction of the previously stored data can not occur on the data storing medium.
As above described, in the optical information recording and reproducing memory system, the light beam emitted from the light source is charged in its power (in other words intensity) depending on the operational modes.
Furthermore, the light beam is, in general, focus controlled and/or tracking-controlled so that the precisely positioned light spot can be directed to the data storing medium. The light spot is generally focused into about 1 micron meter or less in its diameter by means of the focusing control.
The light beam control of the focusing and/or tracking is performed by means of the control system which comprises a control circuit and an optical system with an objective lens, a mirror and other optical components of an optical pickup.
The control system is fundamentally a servo loop control system. The light beam from the light source is transmitted or reflected by the data storing medium, and the light is detected by a dual- or quadruple-photo detector. The detector supplies a servo error signal to the control circuit. The control circuit drives the optical system depending on the servo error signal and the light beam is controlled to effect focusing and/or tracking in a closed loop manner.
To put it more concretely, the focusing control of the light beam is performed by the shift of the moving objective lens or the optical pickup which is driven by the control circuit depending on the focus error signal as a servo error signal. Also the tracking control of the light beam is performed by the shift of the objective lens position, or the change of the inclination of the mirror, or the shift of the optical pickup position by means of a voice coil motor which is driven by the control circuit depending on the tracking error signal as a servo error signal.
However, as previously described, the light source emits the high powered light beam and the weakened low powered light beam in the write mode and the read-out mode, respectively. Therefore, in the prior art, once the loop gain of the control system is adjusted to the write mode, the control system has to work in the poor control gain when the memory system is operated in the read-out mode. On the other hand, once the loop gain of the control system is adjusted to the read-out mode, the control system has to work in the excessive control gain when the memory system is operated in the write mode.
Thus the memory system in the prior art has a shortcoming of unstable control in the light beam focusing or tracking.
To overcome the above drawback, there is proposed an improved optical memory system in which the loop gain of the control system is changed by means of a change-over switch together with the change of the power of the light beam (Japanese Patent Application Laid-open No. 84-22290).
According to the prior art, the loop gain of the control system is changed by means of the switch depending on the operation mode of the memory system e.g. write mode or read-out mode.
However, the control system can not maintain the suitably controlled condition when there is a drop in the power of the light beam due to the degradation of the light source or others. Thus, there is still left the short coming of the poverty of the stable control in the light beam focusing and/or tracking in the memory system.
Besides the prior art above described, there is another prior art for the compensation of the drop in the power of light beam due to the degradation of the light source so as to maintain the suitably controlled condition of the control system.
In the above system, a part of the light beam emitted from the light source is splitted by an optical means e.g. beam splitter which is located in the light pass between the light source and the data storing medium. The intensity of the split light beam is detected by means of an especially located photo detector e.g. silicon photo-cell. Thus the control system can be operated to maintain the suitably controlled condition according to the signal detected by the photo detector (Journal of the SMPTE, vol. 83, pp. 557, 1975).
According to the prior art as above described, however, not only the beam splitter and other components are especially required for the system but also a part of the light beam is split away from the emitted light flux from the light source by means of the beam splitter. Thus the light beam which is originally used for the irradiation onto the data storing medium is partially used for the detecting of the light power. Furthermore it is difficult to shoot the cause of the trouble due to the degradation of the light source or the matter with the optical means.